Ice accumulation is a serious problem for many industries including aerospace, marine, wind energy, power utilities, refrigeration, and commercial fishing. Telecommunications towers are affected in cold environments when icing on exposed structures causes damage. Icing leads to material loss, reduced performance, and interference with normal operations. Icing often leads to injuries and sometimes to deadly accidents.
Because of the broad range of effected sectors, there is no universal solution to ice accumulation. Use of the term “ice-phobic”, which suggests some surfaces prevent ice formation, is incorrect as no coating or surface prevents ice formation under all icing conditions. Depending on the application, the desired outcome is usually the prevention of ice accumulation through easy removal at an early stage of accretion by “natural” forces including wind, vibration, or centrifugal force. The extent of accumulation that can be tolerated varies greatly as does the degree to which ice can be removed from a surface by “natural means”. For throwing office by centrifugal force, a coating technology must take into account that the surface of a wind turbine blade close to the rotor moves much more slowly than the tip of the blade. Power lines are fixed, but may undergo substantial flexing due to wind and vibration.
Perhaps the most demanding applications requirements are those posed by the aerospace industry. These applications have strict requirements for maximum tolerable mass and for uncompromised reliability. It is well known that airfoil icing disrupts airflow, reduces lift, and jeopardizes control. Currently, the aviation industry broadly employs active anti-icing (e.g., heating) to mitigate icing related problems.
Ice accumulation on airplane wings must be removed before takeoff, typically with ethylene or propylene glycol-based fluids or foams. EPA estimated more than 25 million gallons of de-icing agents are annually applied at commercial US airports. De-icing agents are normally not recycled and are discharged directly into waste water systems. Such discharge has caused increased biological oxygen demand and total organic content in groundwater. For the aviation industry, de-icing agents are the method of choice despite the environmental concerns. As more environmentally benign de-icing methods are developed and environmental regulations become stricter, alternatives such as highly efficient ice release coatings will be sought.
Power transmission and telecommunications often encounter problems from icing. In these instances, billion dollar losses can be suffered in major winter storms. In December, 2008, an ice-storm crippled the eastern New England states. The storm impacted an area of 3,250 square miles of the National Grid power company's service area in Massachusetts, New Hampshire and Rhode Island. National Grid had to repair or replace more than 416,000 feet of distribution wire.
The industrial freezer industry has icing problems that are not generally known or appreciated. In commercial freezer facilities, processed foods are transferred into “blast freezer” rooms where liquid dripping from the food forms ice on the floors. Another problem for industrial freezers is ice development around the entrance doors to freezer sections. For this application, manufacturers of refrigeration units seek ice-release coatings that bond well to substrates such as high impact polystyrene. Other problems for which ice-release coatings offer promise include amelioration of blockage of drains and “icing-up” of air conditioners.
From the above summary, market needs for products from which ice can be removed easily vary widely in terms of technical requirements and challenges.
Currently used active methods for de-icing include de-icing fluids for aircraft discussed above and resistive heating where ample power is available such as wind turbines, automobile windshields, and refrigeration units. Resistive heating is costly to implement and reduces net power generated from wind farms. Passive de-icing methods such as icephobic and ice-release coatings are based on silicones or fluoropolymers. Silicones are known for their weak mechanical properties and high cost. Fluorocarbon polymers, if used in the neat form, are even more expensive than silicone materials.
It is logical to think that ice cannot form if water does not wet the surface. Therefore, superhydrophobic surfaces have been investigated to achieve icephobic surfaces. In most cases, such surfaces require careful microstructural fabrication or electrospinning to generate specific complex microstructures for samples that have dimensions of a few square centimeters. Such complex processes are not applicable for large surface areas, at least at present.
A common but mistakenly held notion is that polytetrafluoroethylene (PTFE) or “Teflon” should be good for ice release. Teflon and similar semicrystalline fluoropolymers are processable at high temperatures to generate “non-stick” surfaces for cookware and the like. However, such high temperature processes are not applicable for large area coating technologies. Secondly, polymers made of long fluorocarbon chains (>C6) are degradable to perfluorooctanoic acid (PFOA) that persists indefinitely in the environment. PFOA is bioaccumulative and is a proven carcinogen. Again, current technologies are inadequate.